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United States Patent |
5,327,003
|
Itabashi
,   et al.
|
July 5, 1994
|
Semiconductor static RAM having thin film transistor gate connection
Abstract
A semiconductor memory device includes a semiconductor substrate, a memory
cell provided on the semiconductor substrate and including first and
second transfer transistors, first and second driver transistors and first
and second thin film transistor loads, where each of the first and second
transfer transistors, the first and second driver transistors and the
first and second thin film transistor loads have a source, a drain and a
gate electrode, and a connecting region in which the drain of the second
thin film transistor load, the gate electrode of the first thin film
transistor load and the gate electrode of the first driver transistor are
connected. The gate electrode of the first driver transistor, the gate
electrode of the first thin film transistor load and the drain of the
second thin film transistor load are made of conductor layers which are
stacked on the semiconductor substrate with an insulator layer interposed
between the conductor layers, and a top one of the stacked conductor
layers makes contact with a top surface of a bottom one of the stacked
conductor layers and with side surfaces of each conductor layer provided
between the top and bottom conductor layers within the connecting region.
Inventors:
|
Itabashi; Kazuo (Kawasaki, JP);
Ema; Taiji (Kawasaki, JP)
|
Assignee:
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Fujitsu Limited (Kawasaki, JP)
|
Appl. No.:
|
005186 |
Filed:
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January 15, 1993 |
Foreign Application Priority Data
| Mar 08, 1991[JP] | 3-067707 |
| Jun 18, 1991[JP] | 3-145940 |
Current U.S. Class: |
257/385; 257/67; 257/393; 257/903; 257/E27.098; 257/E27.1 |
Intern'l Class: |
H01L 027/02 |
Field of Search: |
357/59 E,42,71,23.7,41,904
257/67,393,903,385
|
References Cited
U.S. Patent Documents
4764801 | Aug., 1988 | McLaughlin et al. | 257/588.
|
4931410 | Jun., 1990 | Tokunaga et al. | 437/189.
|
5028975 | Jul., 1991 | Nagasawa et al. | 357/41.
|
5034797 | Jul., 1991 | Yamanaka et al. | 257/904.
|
5210429 | May., 1993 | Adan | 257/67.
|
Foreign Patent Documents |
59-231851 | Dec., 1984 | JP | 357/23.
|
62-203363 | Sep., 1987 | JP.
| |
64-82559 | Jun., 1989 | JP.
| |
1-1446556 | Jun., 1989 | JP.
| |
2-312271 | Dec., 1990 | JP.
| |
Other References
Yamanaka et al., "A 25 .mu.m.sup.2 New Poly-Si PMOS Load (PPL) SRAM Cell
Having Excellent Soft Error Immunity," International Electron Devices
Meeting, Dec. 1988, San Francisco, Calif., pp. 48-51.
Ishibashi et al., "An .alpha.-Immune, 2-V Supply Voltage SRAM Using a
Polysilicon PMOS Load Cell," IEEE Journal of Solid-State Circuits, vol.
25, No. 1, Feb. 1990, New York, N.Y., pp. 55-60.
Adan et al., "A Half-Micron SRAM Cell Using a Double-Gated Self-Aligned
Polysilicon PMOS Thin Film Transistor (TFT) Load," Symposium on VLSI
Technology, Jun. 1990, Honolulu, Japan, pp. 19-20.
|
Primary Examiner: Limanek; Robert
Attorney, Agent or Firm: Staas & Halsey
Parent Case Text
This application is a continuation of application Ser. No. 07/846,829,
filed Mar. 6, 1992 now abandoned.
Claims
What is claimed is:
1. A semiconductor memory device comprising:
a semiconductor substrate;
a memory cell provided on the semiconductor substrate and including first
and second transfer transistors, first and second driver transistors and
first and second thin film transistor loads, each of said first and second
transfer transistors, said first and second driver transistors and said
first and second thin film transistor loads having a source, a drain and a
gate electrode;
a connection region in which the drain of the second thin film transistor
load, the gate electrode of the first thin film transistor load and the
gate electrode of the first driver transistor are electrically connected;
the gate electrode of the first driver transistor, the gate electrode of
the first thin film transistor load and the drain of the second thin film
transistor load being made of conductor layers which are stacked in
succession on said semiconductor substrate, a respective insulator layer
being interposed between each two successively stacked conductor layers
and thereby defining a bottom conductor layer and successive, intermediate
and top conductor layers with respective insulation layers interposed
therebetween; and
a surface portion of the bottom conductor layer being exposed in the
connecting region and at least transverse wall surfaces of each
intermediate conductor layer being exposed within the connecting region
and the top conductor layer being connected electrically within, and by,
the connecting region with the exposed top surface portion of the bottom
stacked conductor layer and primarily with the respective transverse wall
surfaces of the intermediate, stacked conductor layers.
2. The semiconductor memory device as claimed in claim 1, wherein the top
conductor layer forms the drain of the second thin film transistor load,
the bottom conductor layer forms the gate electrode of the first driver
transistor, and an intermediate conductor layer, between the top and
bottom conductor layers, forms the gate electrode of the first thin film
transistor load.
3. The semiconductor memory device as claimed in claim 2, wherein at least
the intermediate conductor layers are made of polysilicon, and the
insulator layers are made of SiO.sub.2.
4. The semiconductor memory device as claimed in claim 3, wherein the
bottom conductor layer comprises a first layer formed on said
semiconductor substrate and a second layer formed on the first layer, said
second layer being made of a material selected from the group consisting
of refractory metals and refractory metal silicides.
5. The semiconductor memory device as claimed in claim 2, wherein the drain
of the second driver transistor is further connected to the gate electrode
of the first driver transistor, the gate electrode of the first thin film
transistor load and the drain of the second thin film transistor load by
said connecting region.
6. The semiconductor memory device as claimed in claim 1, wherein the first
thin film transistor load has a channel region, and the gate electrode of
the first thin film transistor load is made up of a lower gate electrode,
which is formed under the channel region and separated therefrom by a
respective, said interposed insulator layer, and an upper gate electrode,
which is formed above the channel region and separated therefrom by a
respective, said interposed insulator layer.
7. The semiconductor memory device as claimed in claim 6, wherein the top
conductor layer forms the upper gate electrode of the first thin film
transistor load, the bottom conductor layer forms the gate electrode of
the first driver transistor, and the conductor layers provided between the
top and bottom conductor layers respectively form the drain of the second
thin film transistor load and the lower gate electrode of the first thin
film transistor load.
8. The semiconductor memory device as claimed in claim 7, wherein at least
each intermediate conductor layer is made of polysilicon and the insulator
layers are made of SiO.sub.2.
9. The semiconductor memory device as claimed in claim 8, wherein the
bottom conductor layer comprises a first layer formed on said
semiconductor substrate and a second layer formed on the first layer, said
second layer being made of a material selected from the group consisting
of refractory metals and refractory metal silicides.
10. The semiconductor memory device as claimed in claim 7, wherein the
drain of the second driver transistor is further connected to the gate
electrode of the first driver transistor, the gate electrode of the first
thin film transistor load and the drain of the second thin film transistor
load by said connecting region.
11. A semiconductor memory device comprising:
a semiconductor substrate;
a memory cell provided on the semiconductor substrate and including first
and second transfer transistors, first and second driver transistors and
first and second thin film transistor loads, each of said first and second
transfer transistors, said first and second driver transistors and said
first and second thin film transistor loads having a source, a drain and a
gate electrode;
a connection region in which the drain of the second thin film transistor
load, the gate electrode of the first thin film transistor load and the
gate electrode of the first driver transistor are electrically connected;
the gate electrode of the first driver transistor, the gate electrode of
the first thin film transistor load and the drain of the second thin film
transistor load being made of conductor layers which are stacked in
succession on said semiconductor substrate and a respective insulator
layer is interposed between each two successive stacked conductor layers
and thereby defining a bottom conductor layer and successive, intermediate
and top conductor layers with respective insulation layers interposed
therebetween; and
the connecting region further comprising:
a contact window extending through the successively stacked top and
intermediate conductor layers and each interposed insulator layer and
exposing a surface portion of the bottom conductor layer, respective
transverse wall surfaces of the stacked conductor layers and interposed
insulator layers being exposed at and defining corresponding sidewalls of
the contact hole, the respective transverse wall surfaces of the stacked
conductor layers and the interposed insulator layers, corresponding to
each sidewall of the contact hole, being substantially aligned, and
a conductor material formed within and covering at least the sidewalls of
the contact window and the exposed top surface portion of the bottom
conductor layer, the conductor material being formed on at least the
exposed transverse wall surfaces of the respective conductor layers and
electrically connecting the top conductor layer with the intermediate
conductor layers and the bottom layer.
12. The semiconductor memory device as claimed in claim 11, wherein the top
conductor layer forms the drain of the second thin film transistor load,
the bottom conductor layer forms the gate electrode of the first driver
transistor, and an intermediate conductor layer, between the top and
bottom conductor layers, forms the gate electrode of the first thin film
transistor load.
13. The semiconductor memory device as claimed in claim 12, wherein at
least the intermediate conductor layers are made of polysilicon and the
insulator layers are made of SiO.sub.2.
14. The semiconductor memory device as claimed in claim 13, wherein the
bottom conductor layer comprises a first layer formed on said
semiconductor substrate and a second layer formed on the first layer, said
second layer being made of a material selected from the group consisting
of refractory metals and refractory metal silicides.
15. The semiconductor memory device as claimed in claim 12, wherein the
drain of the second driver transistor is further connected to the gate
electrode of the first driver transistor, the gate electrode of the first
thin film transistor load and the drain of the second thin film transistor
load by said connecting region.
16. The semiconductor memory device as claimed in claim 11, wherein the
first thin film transistor load has a channel region, and the gate
electrode of the first thin film transistor load is made up of a lower
gate electrode, which is formed under the channel region and separated
therefrom by a respective, said interposed insulator layer, and an upper
gate electrode, which is formed above the channel region and separated
therefrom by a respective, said interposed insulator layer.
17. The semiconductor memory device as claimed in claim 16, wherein the top
conductor layer forms the upper gate electrode of the first thin film
transistor load, the bottom conductor layer forms the gate electrode of
the first driver transistor, and the conductor layers provided between the
top and bottom conductor layers respectively form the drain of the second
thin film transistor load and the lower gate electrode of the first thin
film transistor load.
18. The semiconductor memory device as claimed in claim 7, wherein at least
each intermediate conductor layer is made of polysilicon and the insulator
layers are made of SiO.sub.2.
19. The semiconductor memory device as claimed in claim 8, wherein the
bottom conductor layer comprises a first layer formed on said
semiconductor substrate and a second layer formed on the first layer, said
second layer being made of a material selected from the group consisting
of refractory metals and refractory metal silicides.
20. The semiconductor memory device as claimed in claim 7, wherein the
drain of the second driver transistor is further connected to the gate
electrode of the first driver transistor, the gate electrode of the first
thin film transistor load and the drain of the second thin film transistor
load by said connecting region.
21. The semiconductor memory device as recited in claim 18, wherein the
transverse wall surfaces of the conductor layers protrude inwardly of the
transverse wall surfaces of the corresponding insulator layers at each
sidewall of the contact hole, the conductor material, further, being
formed on and connected to the protruding surfaces of the intermediate
conductor layers.
Description
BACKGROUND OF THE INVENTION
The present invention generally relates to semiconductor memory devices and
methods of producing the same, and more particularly to a thin film
transistor (TFT) load type static random access memory (SRAM) and a method
of producing such a TFT load type SRAM.
Up to now, the high resistance load type SRAM was popularly used. However,
as the integration density improves and the number of memory cells
increases, the current consumption increases and various problems are
generated. In order to avoid such problems and the help of the progress in
the semiconductor technology, the SRAM having the TFT load has been
realized. However, new problems are generated due to the use of the TFT
load, and it is necessary to eliminate these new problems.
An example of a conventional method of producing the high resistance load
type SRAM will be described with reference to FIGS.1A through 1J and FIGS.
2A through 2F. FIGS. 1A through 1J are side views in cross section showing
essential parts of the high resistance load type SRAM at essential stages
of the conventional method of producing the high resistance load type
SRAM. FIGS. 2A through 2F are plan views of the high resistance load type
SRAM at essential stages of the conventional method of producing the high
resistance load type SRAM. FIGS.1A through 1J respectively are cross
sections taken along a line which corresponds to a line Y--Y in the plan
view of FIG.2F.
In FIG.1A, a silicon dioxide (SiO.sub.2) layer is used as a pad layer, for
example, and a silicon nitride (Si.sub.3 N.sub.4) layer which is formed on
the SiO.sub.2 layer is used as an oxidation resistant mask layer when
carrying out a selective thermal oxidation (for example, a local oxidation
of silicon (LOCOS)) so as to form a field insulator layer 2 on a silicon
(Si) semiconductor substrate 1. This field insulator layer 2 is made of
SiO.sub.2 and has a thickness of 4000 .ANG., for example.
Then, the Si.sub.3 N.sub.4 layer and the SiO.sub.2 layer which are used
when carrying out the selective thermal oxidation are removed to expose an
active region of the Si semiconductor substrate 1.
In FIG.1B, a thermal oxidation is carried out to form a gate insulator
layer 3 which is made of SiO.sub.2 and has a thickness of 100 .ANG., for
example.
By carrying out a resist process of the photolithography technique and a
wet etching using hydrofluoric acid as the etchant, the gate insulator
layer 3 is selectively etched to form a contact hole 3A.
In FIGS.1C and 2A, a chemical vapor deposition (CVD) is carried out to form
a first polysilicon layer having a thickness of 1500 .ANG., for example.
Then, a vapor phase diffusion is carried out to introduce phosphorus (P) of
1.times.10.sup.20 cm.sup.-3, for example, so as to form an n.sup.+ -type
impurity region 5'.
In FIG.2A, the illustration of the first polysilicon layer is omitted for
the sake of convenience.
In FIG.1D, a resist process of the photolithography technique and a
reactive ion etching (RIE) using CCl.sub.4 /O.sub.2 as the etching gas are
carried out to pattern the first polysilicon layer and form a gate
electrode 4. The gate electrode 4 becomes the gate electrode of a word
line and a driver transistor.
An ion implantation is carried out to inject As ions with a dosage of
3.times.10.sup.15 cm.sup.-2 and an acceleration energy of 40 keV, so as to
form a source region 5 and a drain region 6.
In FIGS.1E and 2B, a CVD is carried out to form an insulator layer 7 which
is made of SiO.sub.2 and has a thickness of 1000 .ANG., for example.
By carrying out a resist process of the photolithography technique and a
RIE using CHF.sub.3 /He as the etching gas, a ground line contact hole 7A
is formed. This ground line contact hole 7A cannot be seen in FIG.1E.
In FIG.1F, a CVD is carried out to form a second polysilicon layer having a
thickness of 1500 .ANG., for example.
Then, an ion implantation is carried out to inject P ions into the second
polysilicon layer with a dosage of 4.times.10.sup.15 cm.sup.-2 and an
acceleration energy of 30 keV, and an annealing is carried out to reduce
the resistance.
A resist process of the photolithography technique and a RIE using
CCl.sub.4 /O.sub.2 as the etching gas are carried out to pattern the
second polysilicon layer and form a ground line 8.
In FIGS.1G and 2C, a CVD is carried out to form an insulator layer 9 which
is made of SiO.sub.2 and has a thickness of 1000 .ANG., for example.
A resist process of the photolithography technique and a RIE using
CHF.sub.3 /He as the etching gas are carried out to selectively etch the
insulator layers 9 and 7 and form a load resistor contact hole 9A.
In FIG.1H, a CVD is carried out to form a third polysilicon layer having a
thickness of 1500 .ANG., for example.
A resist process of the photolithography technique and an ion implantation
with a dosage of 1.times.10.sup.15 cm.sup.-2 and an acceleration energy of
30 keV are carried out to inject As ions into a part where a supply line
of a positive power source voltage Vcc is formed and a part where the high
resistance load makes contact with the gate electrode 4.
By carrying out a resist process of the photolithography technique and a
RIE using CCl.sub.4 /O.sub.2 as the etching gas, the third polysilicon
layer is patterned to form a contact part 10, a high resistance load 11
and a Vcc supply line 12.
In FIGS.1I and 2D, a CVD is carried out to form an insulator layer which is
made of SiO.sub.2 and has a thickness of 1000 .ANG., for example, and an
insulator layer which is made of phosphosilicate glass (PSG) and has a
thickness of 5000 .ANG., for example. In FIG.1I, these insulator layers
are referred to as an insulator layer 13.
A thermal process is thereafter carried out to reflow and planarize the
insulator layer 13.
Next, a resist process of the photolithography technique and a RIE using
CHF.sub.3 /He as the etching gas are carried out to selectively etch the
insulator layer 13 and the like and to form a bit line contact hole 13A.
In FIGS.1J and 2E, a sputtering is carried out to form an aluminum (A1)
layer having a thickness of 1 .mu.m, for example. This A1 layer is
patterned using the normal photolithography technique so as to form a bit
line 14. Those elements which are shown in FIGS.1J and 2E but not yet
described, such as "BL", will be readily understood from the description
given later in conjunction with FIG.3.
FIG.2F shows the plan view of the essential part of the high resistance
load type SRAM which is completed by the above described processes. In
FIG.2F, those parts which are the same as those corresponding parts in
FIGS.1A through 1J and FIGS.2A through 2E are designated by the same
reference numerals. However, for the sake of convenience, the illustration
of the A1 bit line 14 shown in FIGS.1J and 2E is omitted in FIG.2F.
FIG.3 shows an equivalent circuit diagram of the essential part of the high
resistance load type SRAM described above in conjunction with FIGS.1A
through 1J and 2A through 2F.
FIG.3 shows driver transistors Q1 and Q2, transfer gate transistors Q3 and
Q4, high resistance loads R1 and R2, a word line WL, bit lines BL and BL,
nodes S1 and S2, the positive power source voltage Vcc, and a negative
power source voltage Vss.
The operation of this high resistance load type SRAM, the storage operation
in particular, is carried out as follows.
If it is assumed that the positive power source voltage Vcc is 5 V, the
negative power source voltage Vss is 0 V, the node S1 is 5 V and the node
S2 is 0V, the transistor Q2 is ON and the transistor Q1 is OFF. The
potential at the node S1 is maintained to 5 V if the transistor Q1 is OFF
and the resistance is sufficiently high compared to the high resistance
load R1. The potential at the node S2 is maintained to 0 V if the
transistor Q2 is ON and the resistance is sufficiently low compared to the
high resistance load R2.
However, under the above described condition, a D.C. current flows from the
positive power source voltage Vcc supply line to the negative power source
voltage Vss supply line via the node S2, and the current value is
inversely proportional to the value of the high resistance load R2.
When the integration density of the above described high resistance load
type SRAM increases, the number of memory cells per chip increases and the
current consumption of the entire chip would become very large if the
current consumption per memory is not reduced. Hence, the D.C. current
described above must be reduced, but in order to reduce this D.C. current,
the values of the high resistance loads R1 and R2 must be set large.
However, when the values of the high resistance loads R1 and R2 are set
large, it becomes difficult to stably maintain the potential at the nod
having the driver transistor which is OFF, that is, the potential at the
node S1 in FIG.3.
Because of the above described background, the TFT load type SRAM which
uses the TFT as the load in place of the high resistance load has been
developed.
Next, a description will be given of the TFT load type SRAM. Similarly to
the description given above in respect of the high resistance load type
SRAM, a description will first be given of the method of producing the TFT
load type SRAM.
An example of a conventional method of producing the TFT load type SRAM
will be described with reference to FIGS.4A through 4D and FIGS.5A through
5D. FIGS.4A through 4D are side views in cross section showing essential
parts of the TFT load type SRAM at essential stages of the conventional
method of producing the high resistance load type SRAM. FIGS.5A through 5D
are plan views of the TFT load type SRAM at essential stages of the
conventional method of producing the TFT load type SRAM. FIGS.4A through
4D respectively are cross sections taken along a line which corresponds to
a line Y--Y in the plan view of FIG.5D.
The processes of producing the TFT load type SRAM at the beginning are
basically the same as the processes described in conjunction with FIGS.1A
through 1G up to the process of forming the load resistor contact hole 9A
of the high resistance load type SRAM, and a description thereof will be
omitted. The only difference is that a contact hole 8A shown in FIG.5A is
formed with respect to the ground line 8 which is made of the second
polysilicon layer, so that a gate electrode of a TFT which is formed by a
third polysilicon layer can make contact with an active region and the
gate electrode 4 which is formed by the first polysilicon layer. Hence, a
description will only be given from the processes thereafter. In FIGS.4A
through 4D and 5A through 5D, those parts which are the same as those
corresponding parts in FIGS.1A through 1J and 2A through 2F are designated
by the same reference numerals.
In FIGS.4A and 5A, a CVD is carried out to form a third polysilicon layer
having a thickness of 1500 .ANG., for example.
Then, an ion implantation is carried out to inject P ions with a dosage of
4.times.10.sup.15 cm.sup.-2 and an acceleration energy of 30 keV.
Further, a resist process of the photolithography technique and a RIE using
CCl.sub.4 /O.sub.2 as the etching gas are carried out to pattern the third
polysilicon layer and form a gate electrode 15 of the TFT.
In FIG.4B, a CVD is carried out to form a gate insulator layer 16 of the
TFT, which is made of SiO.sub.2 and has a thickness of 300 .ANG., for
example.
A resist process of the photolithography technique and a wet etching using
hydrofluoric acid as the etchant are carried out to selectively etch the
gate insulator layer 16 and form a drain contact hole 16A.
In FIGS.4C and 5B, a CVD is carried out to form a fourth polysilicon layer
having a thickness of 500 .ANG., for example.
A resist process of the photolithography technique and an ion implantation
with a dosage of 1.times.10.sup.14 cm.sup.-2 and an acceleration energy of
5 keV are carried out to inject B ions into a part where a supply line of
a positive power source voltage Vcc is made and into a part where the TFT
source and drain regions are formed.
A resist process of the photolithography technique and a RIE using
CCl.sub.4 /O.sub.2 as the etching gas are carried out to pattern the
fourth polysilicon layer and form a source region 17, a drain region 18
and a channel region 19 of the TFT and also form a Vcc supply line 20.
In FIGS.4D and 5C, a CVD is carried out to form an insulator layer made of
SiO.sub.2 and having a thickness of 1000 .ANG., for example, and an
insulator layer made of PSG and having a thickness of 5000 .ANG., for
example. In FIG.4D, these two insulator layers are shown as one insulator
layer 21, similarly as in the case of FIGS.1I and 1J.
Then, a thermal process is carried out to reflow and planarize the
insulator layer 21.
Next, a resist process of the photolithography technique and a RIE using
CHF.sub.3 /He as the etching gas are carried out to selectively etch the
insulator layer 21 and the like and to form a bit line contact hole.
A sputtering is carried out thereafter to form an A1 layer having a
thickness of 1 .mu.m, for example, and this A1 layer is patterned by the
normal photolithography technique to form a bit line 22. Those elements
which are shown in FIGS.4D and 5C but not yet described, such as "BL",
will be readily understood from the description given later in conjunction
with FIG.6.
FIG.5D shows the plan view of the essential part of the TFT load type SRAM
which is completed by the above described processes. In FIG.5D, those
parts which are the same as those corresponding parts in FIGS.4A through
4D and FIGS.5A through 5D are designated by the same reference numerals.
However, for the sake of convenience, the illustration of the A1 bit line
22 shown in FIGS.4D and 5C is omitted in FIG.5D.
FIG.6 shows an equivalent circuit diagram of an essential part of the TFT
load type SRAM described in conjunction with FIGS.4A through 4D and 5A
through 5D. In FIG.6, those parts which are the same as those
corresponding parts in FIGS.4A through 4D and 5A through 5D are designated
by the same reference numerals.
FIG.6 shows transistors Q5 and Q6 which are load TFTs used in place of the
high resistance loads R1 and R2 shown in FIG.3.
Next, a description will be given of the operation of the TFT load type
SRAM, and the storing operation in particular.
If it is assumed that the positive power source voltage Vcc is 5 V, the
negative power source voltage Vss is 0 V, the node S1 is 5 V and the node
S2 is 0 V, the transistor Q6 is OFF when the transistor Q2 is ON and the
transistor Q5 is ON when the transistor Q1 is OFF. The potential at the
node S1 is maintained to 5 V if the transistor Q1 is OFF and the
resistance is sufficiently high compared to the transistor Q5 which is ON.
The potential at the node S2 is maintained to 0 V if the transistor Q2 is
ON and the resistance is sufficiently small compared to the transistor Q6
which is ON.
Under the above described condition, the resistance of the load transistor
Q5 or Q6 changes depending on the stored information, and thus, the
problems of the high resistance load type SRAM is eliminated. That is, it
is possible to carry out a stable information storage operation. The
channels of the transistors Q5 and Q6, that is, the channels of the load
TFTs, are made of polysilicon. The crystal state of the polysilicon which
forms the channels is considerably poor compared to the single crystal,
and a current easily leaks even when the transistor is OFF. Such a leak
current increases the current consumption of the chip, and it is desirable
to make the channel as small as possible.
On the other hand, as may be readily seen from FIG.4D, the bit line 22
which is made of the A1 layer is provided at the top layer of the TFT load
type SRAM. In addition, the channel of the load TFT exists immediately
under the bit line 22 via the insulator layer 21 which is made of PSG or
the like.
But according to this construction, the bit line 22 which is made of the A1
layer can be regarded as a gate electrode of a transistor, and the
underlying insulator layer 21 can be regarded as a gate insulator layer of
this transistor. In addition, the potential of the bit line 22 which is
regarded as the gate electrode varies between 0 v (Vss) and 5 V (Vcc). As
a result, the TFT which should be OFF, that is, the transistor Q6 becomes
nearly ON, and the leak current increases and the parasitic effect becomes
notable.
Accordingly, a double gate structure TFT load type SRAM was developed in
order to eliminate the above described problems of the TFT load type SRAM.
According to the double gate structure TFT load type SRAM, the above
described problems of the TFT load type SRAM are eliminated by interposing
the third polysilicon layer of the TFT load type SRAM described in
conjunction with FIGS.4 through 6 between the fourth polysilicon layer and
the bit line 22 which is made of A1. Particularly, a fifth polysilicon
layer forming a second gate electrode which has the same pattern as the
gate electrode 15 of the TFT is interposed between the A1 bit line 22 and
the fourth polysilicon layer which forms the source region 17, the drain
region 18, the channel region 19, the Vcc supply line 20 and the like.
FIGS.7A through 7C are side views in cross section showing essential parts
of the double gate structure TFT load type SRAM at essential stages of the
conventional method of producing the double gate structure TFT load type
SRAM. The processes of producing the double gate structure TFT load type
SRAM at the beginning are basically the same as the processes described in
conjunction with FIGS.4A through 4C up to the process of forming the
source region 17, the drain region 18, the channel region 19 and the Vcc
supply line 20 of the TFT load type SRAM, and a description thereof will
be omitted. Hence, a description will only be given from the processes
thereafter. In FIGS.7A through 7C, those parts which are the same as those
corresponding parts in FIGS.1 through 6 are designated by the same
reference numerals.
In FIG.7A, a CVD is carried out to form an insulator layer 23 which is made
of SiO.sub.2 and has a thickness of 500 .ANG., for example.
A resist process of the photolithography technique and a RIE using
CHF.sub.3 +He as the etching gas are carried out to selectively etch the
insulator layer 23 and to form a contact hole 23A with respect to the
drain electrode 18 of the TFT.
In FIG.7B, a CVD is carried out to form a fifth polysilicon layer having a
thickness of 1000 .ANG., for example.
Then, an ion implantation is carried out to inject P ions into the fifth
polysilicon layer with a dosage of 4.times.10.sup.15 cm.sup.-2, for
example.
A resist process of the photolithography technique and a RIE using
CCl.sub.4 /O.sub.2 as the etching gas are carried out to pattern the fifth
polysilicon layer and to form a second gate electrode 24 of the TFT.
In FIG.7C, a CVD is carried out to form an insulator layer which is made of
SiO.sub.2 and has a thickness of 1000 .ANG., for example, and an insulator
layer which is made of PSG and has a thickness of 5000 .ANG., for example.
As in the case shown in FIG.4D, these two insulator layers are shown as
one insulator layer 25 in FIG.7C.
Thereafter, a thermal process is carried out to reflow and planarize the
insulator layer 25.
Next, a resist process of the photolithography technique and a RIE using
CHF.sub.3 /He as the etching gas are carried out to selectively etch the
insulator layer 25 and the like, and to form a bit line contact hole.
In addition, a sputtering is carried out to form an A1 layer having a
thickness of 1 .mu.m, for example, and this A1 layer is patterned by the
normal photolithography technique so as to form a bit line 26.
As described heretofore, the SRAM started from the high resistance load
type, evolved to the TFT load type, and further evolved to the double gate
structure TFT load type. However, as may be seen by comparing FIGS.1A
through 1J with FIGS.7A through 7C, and FIGS.1J and 7C in particular, the
number of polysilicon layers has increased by two from the high resistance
load type SRAM to the double gate structure TFT load type SRAM, and the
number of mask processes have increased by four.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of the present invention to provide a
novel and useful thin film transistor (TFT) load type static random access
memory (SRAM) in which the problems described above are eliminated.
Another and more specific object of the present invention is to provide a
semiconductor memory device comprising a semiconductor substrate, a memory
cell provided on the semiconductor substrate and including first and
second transfer transistors, first and second driver transistors and first
and second thin film transistor loads, where each of the first and second
transfer transistors, the first and second driver transistors and the
first and second thin film transistor loads have a source, a drain and a
gate electrode, and a connecting region in which the drain of the second
thin film transistor load, the gate electrode of the first thin film
transistor load and the gate electrode of the first driver transistor are
connected, where the gate electrode of the first driver transistor, the
gate electrode of the first thin film transistor load and the drain of the
second thin film transistor load are made of conductor layers which are
stacked on the semiconductor substrate with an insulator layer interposed
between the conductor layers, and a top one of the stacked conductor
layers makes contact with a top surface of a bottom one of the stacked
conductor layers and with side surfaces of each conductor layer provided
between the top and bottom conductor layers within the connecting region.
According to the semiconductor memory device of the present invention, the
connection of the driver transistor and the thin film transistor load in
the connecting region can be achieved by forming a contact hole in one
process. Hence, the number of mask processes can be reduced by one
compared to the conventional method of producing the TFT load type SRAM,
and by two compared to the conventional method of producing the double
gate structure TFT load type SRAM. Therefore, the TFT load type SRAM can
be produced with ease using simple processes and with a high production
yield.
Still another object of the present invention is to provide a method of
producing a semiconductor memory device which includes a memory cell made
up of two transfer transistors, two driver transistors and two thin film
transistor loads, which method comprises the steps of (a) forming a gate
insulator layer on a semiconductor substrate after forming a field
insulator layer on a surface of the semiconductor substrate, (b) forming a
gate electrode of the driver transistor by forming a first conductor layer
and patterning the first conductor layer, (c) forming a first insulator
layer after forming impurity regions in the semiconductor substrate using
the field insulator layer and the first conductor layer as masks, (d)
forming a second insulator layer after forming and patterning a second
conductor layer, (e) forming a contact hole which extends from a top
surface of the second insulator layer and reaches a top surface of the
first conductor layer, and (f) forming a third conductor layer which makes
contact with the top surface of the first conductor layer and with side
surfaces of the second conductor layer. According to the method of the
present invention, the connection of the driver transistor and the thin
film transistor load can be achieved by forming the contact hole in one
process. Hence, the number of mask processes can be reduced by one
compared to the conventional method of producing the TFT load type SRAM.
Therefore, the TFT load type SRAM can be produced with ease using simple
processes and with a high production yield.
A further object of the present invention is to provide a method of
producing a semiconductor memory device which includes a memory cell made
up of two transfer transistors, two driver transistors and two thin film
transistor loads, which method comprises the steps of (a) forming a gate
insulator layer on a semiconductor substrate after forming a field
insulator layer on a surface of the semiconductor substrate, (b) forming a
gate electrode of the driver transistor by forming a first conductor layer
and patterning the first conductor layer, (c) forming a first insulator
layer after forming impurity regions in the semiconductor substrate using
the field insulator layer and the first conductor layer as masks, (d)
forming a second insulator layer after forming and patterning a second
conductor layer, (e) forming a third insulator layer after forming and
patterning a third conductor layer, (f) forming a contact hole which
extends from a top surface of the third insulator layer and reaches a top
surface of the first conductor layer, and (g) forming a fourth conductor
layer which makes contact with the top surface of the first conductor
layer and with side surfaces of the second and third conductors layer.
According to the method of the present invention, the connection of the
driver transistor and the thin film transistor load can be achieved by
forming the contact hole in one process. Hence, the number of mask
processes can be reduced by two compared to the conventional method of
producing the double gate structure TFT load type SRAM. Therefore, the TFT
load type SRAM can be produced with ease using simple processes and with a
high production yield.
Another object of the present invention is to provide a method of producing
the semiconductor memory device of either one of the types described
above, wherein wherein the step (b) includes the substeps of forming a
first layer which is made of a conductive material, and forming a second
layer which is made of a conductive material on the first layer to form
the first conductor layer which is made up of the first and second layers,
where the second layer is made of a material selected from a group
consisting of refractory metals and refractory metal silicides. According
to the method of the present invention, it is possible to prevent damage
to the first conductor layer and the semiconductor substrate including the
impurity regions thereof. Further, it is possible to prevent and etching
residue from remaining within the contact hole when the contact hole is
formed.
Other objects and further features of the present invention will be
apparent from the following detailed description when read in conjunction
with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A through 1J are side views in cross section showing essential parts
of a high resistance load type SRAM at essential stages of a conventional
method of producing the high resistance load type SRAM;
FIGS. 2A through 2F are plan views of the high resistance load type SRAM at
essential stages of the conventional method of producing the high
resistance load type SRAM;
FIG. 3 shows an equivalent circuit diagram of the essential part of the
high resistance load type SRAM described in conjunction with FIGS.1A
through 1J and 2A through 2F;
FIGS.4A through 4D are side views in cross section showing essential parts
of a TFT load type SRAM at essential stages of a conventional method of
producing the TFT load type SRAM;
FIGS.5A through 5D are plan views of the TFT load type SRAM at essential
stages of the conventional method of producing the TFT load type SRAM;
FIG.6 shows an equivalent circuit diagram of an essential part of the TFT
load type SRAM described in conjunction with FIGS.4A through 4D and 5A
through 5D;
FIGS.7A through 7C are side views in cross section showing essential parts
of a double gate structure TFT load type SRAM at essential stages of a
conventional method of producing the double gate structure TFT load type
SRAM;
FIGS.8A through 8D are side views in cross section showing an essential
part of a first embodiment of a semiconductor memory device according to
the present invention at essential stages of a first embodiment of a
method of producing the semiconductor memory device according to the
present invention;
FIGS.9A through 9D are side views in cross section showing an essential
part of a second embodiment of the semiconductor memory device according
to the present invention at essential stages of a second embodiment of the
method of producing the semiconductor memory device according to the
present invention;
FIG.10A through 10F are side views in cross section showing essential parts
of a semiconductor memory device at essential stages of the production,
for explaining the process of forming a contact hole which penetrates a
stacked structure;
FIGS.11A through 11F are side views in cross section showing an essential
part of a third embodiment of the semiconductor memory device according to
the present invention at essential stages of a third embodiment of the
method of producing the semiconductor memory device according to the
present invention, for explaining the operating principle of the third
embodiment;
FIGS.12A through 12H are side views in cross section showing an essential
part of the third embodiment of the semiconductor memory device according
to the present invention at essential stages of the third embodiment of
the method of producing the semiconductor memory device according to the
present invention; and
FIGS.13A through 13D are side views in cross section showing an essential
part of a fourth embodiment of the semiconductor memory device according
to the present invention at essential stages of a fourth embodiment of the
method of producing the semiconductor memory device according to the
present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIGS.8A through 8D are side views in cross section showing an essential
part of a first embodiment of a semiconductor memory device according to
the present invention at essential stages of a first embodiment of a
method of producing the semiconductor memory device according to the
present invention. In this embodiment, the present invention is applied to
the TFT load type SRAM.
In this embodiment of the method, the processes are the same as those of
the prior art method up to the processes shown in FIGS.1A through 1F, that
is, until a ground line 8 is formed from the second polysilicon layer.
Hence, a description will only be given of the processes carried out
thereafter. In the following description, the prior art processes descried
with reference to FIGS.4A through 4D will help the understanding of this
embodiment.
In FIGS.8A through 8D, those parts which are the same as those
corresponding parts in FIGS.1A through 1J are designated by the same
reference numerals, and a description thereof will be omitted.
In FIG.8A, a TFT load type SRAM includes a Si semiconductor substrate 1, a
field insulator layer 2 formed on the Si semiconductor substrate 1, a gate
insulator layer 3, a gate electrode 4 of the driver transistor which is
made up of the first polysilicon layer, an n.sup.+ -type impurity region
5', an n.sup.+ -type source region 5, an n.sup.+ -type drain region 6, an
insulator layer 7, and a ground line 8 which is made up of the second
polysilicon layer.
A CVD is carried out to form an insulator layer 9 which is made of
SiO.sub.2 and has a thickness of 1000 .ANG., for example, on the entire
top surface of the stacked structure.
A CVD is carried out to form a third polysilicon layer having a thickness
of 500 .ANG., for example.
Then, an ion implantation is carried out to inject P ions into the third
polysilicon layer with a dosage of 1.times.10.sup.15 cm.sup.-2 and an
acceleration energy of 10 keV.
In addition, a resist process of the photolithography technique and a RIE
using CCl.sub.4 /O.sub.2 as the etching gas are carried out to pattern the
third polysilicon layer and to form a gate electrode 15 of the TFT.
In FIG.8B, a CVD is carried out to form a gate insulator layer 16 of the
TFT, which is made of SiO.sub.2 and has a thickness of 200 .ANG., for
example.
A resist process of the photolithography technique and RIEs using CHF.sub.3
/He and CCl.sub.4 /O.sub.2 as etching gases for respectively etching
SiO.sub.2 and polysilicon are carried out, to selectively etch the gate
insulator layer 16, the gate electrode 15 which is formed from the third
polysilicon layer, the insulator layer 9 and the insulator layer 7, and to
form a contact hole 16A. This contact hole 16A extends from the surface of
the gate insulator layer 16 and reaches the gate electrode 4 of the
driving transistor made of the first polysilicon layer. This process forms
an essential part of this embodiment.
In FIG.8C, a CVD is carried out to form a fourth polysilicon layer having a
thickness of 200 .ANG., for example.
A resist process of the photolithography technique and an ion implantation
are carried out to inject B ions with a dosage of 1.times.10.sup.14
cm.sup.-2 and an acceleration energy of 5 keV into parts where source and
drain regions of the TFT are formed.
Then, a resist process of the photolithography technique and a RIE using
CCl.sub.4 /O.sub.2 as the etching gas are carried out to pattern the
fourth polysilicon and to form a source region 17, a drain region 18 and a
channel region 19 of the TFT and a Vcc supply line. The Vcc supply line
cannot be seen in FIG.8C.
In FIG.8D, a CVD is carried out to form an insulator layer which is made of
SiO.sub.2 and has a thickness of 1000 .ANG., for example, and an insulator
layer which is made of PSG and has a thickness of 5000 .ANG., for example.
In FIG.8D, these two insulator layers are shown as one insulator layer 21.
Thereafter, a thermal process is carried out to reflow and planarize the
insulator layer 21.
A resist process of the photolithography technique and a RIE using
CHF.sub.3 /He as the etching gas are carried out to selectively etch the
insulator layer 21 and the like and to form a bit line contact hole.
An A1 layer having a thickness of 1 .mu.m, for example, is formed by a
sputtering process, and this A1 layer is patterned using the normal
photolithography technique to form a bit line 22.
As is evident from the description given above with reference to FIGS.8A
through 8C, the contact of the gate electrode of the driver transistor,
the gate electrode of the TFT load and the drain of the TFT load is
achieved by a single mask process. On the other hand, in the case of the
prior art method described with reference to FIGS.4 and 5, two mask
processes are required to achieve this contact.
Next, a description will be given of a second embodiment of the
semiconductor memory device according to the present invention which is
produced by a second embodiment of the method of producing the
semiconductor memory device according to the present invention, by
referring to FIGS.9A through 9D.
FIGS.9A through 9D are side views in cross section showing an essential
part of the second embodiment of the semiconductor memory device according
to the present invention at essential stages of the second embodiment of
the method of producing the semiconductor memory device according to the
present invention. In this embodiment, the present invention is applied to
the double gate structure TFT load type SRAM.
In this embodiment of the method, the processes are the same as those of
the first embodiment of the method up to the processes shown in FIG.8A,
that is, until a gate electrode 15 of the TFT load is formed from the
third polysilicon layer. Hence, a description will only be given of the
processes carried out thereafter. In the following description, the prior
art processes described with reference to FIGS.7A through 7C will help the
understanding of this embodiment.
In FIGS.9A through 9D, those parts which are the same as those
corresponding parts in FIGS.7A through 7C and FIGS.8A through 8D are
designated by the same reference numerals, and a description thereof will
be omitted.
In FIG.9A, a double gate structure TFT load type SRAM includes a Si
semiconductor substrate 1, a field insulator layer 2 formed on the Si
semiconductor substrate 1, a gate insulator layer 3, a gate electrode 4 of
the driver transistor which is made up of the first polysilicon layer, an
n.sup.+ -type impurity region 5', an n.sup.+ -type source region 5, an
n.sup.+ -type drain region 6, an insulator layer 7, a ground line 8 which
is made up of the second polysilicon layer, a lower gate electrode 15 of
the TFT, and a gate insulator layer 16 of the TFT.
A CVD is carried out to form a fourth polysilicon layer having a thickness
of 200 .ANG., for example.
A resist process of the photolithography technique and an ion implantation
are carried out to inject B ions with a dosage of 1.times.10.sup.14
cm.sup.-2 and an acceleration energy of 5 keV into parts where source and
drain regions of the TFT are formed.
A resist process of the photolithography technique and a RIE using
CCl.sub.4 /O.sub.2 as the etching gas are carried out to pattern the
fourth polysilicon layer and to form a source region 17, a drain region 18
and a channel region 19 of the TFT and a Vcc supply line. The Vcc supply
line cannot be seen in FIG.9A.
In FIG.9B, a CVD is carried out to form an insulator layer 23 which is made
of SiO.sub.2 and has a thickness of 500 .ANG., for example.
A resist process of the photolithography technique and RIEs using CHF.sub.3
/He and CCl.sub.4 /O.sub.2 as the etching gases are respectively carried
out to etch the SiO.sub.2 and the polysilicon, to selectively etch the
insulator layer 23, the drain region 18 of the TFT load formed by the
fourth polysilicon, the gate insulator layer 16, the gate electrode 15
formed by the third polysilicon layer, the insulator layer 9 and the
insulator layer 7, and to form a contact hole 23A. This contact hole 23A
extends from the surface of the insulator layer 23 and reaches the surface
of the gate electrode 4 of the driving transistor made of the first
polysilicon. This process forms an essential part of this embodiment.
In FIG.9C, a CVD is carried out to form a fifth polysilicon layer having a
thickness of 500 .ANG., for example.
Then, a thermal diffusion is carried out to diffuse P into the fifth
polysilicon with an impurity concentration of 1.times.10.sup.21 cm.sup.-3,
for example.
A resist process of the photolithography technique and a RIE using
CCl.sub.4 /O.sub.2 as the etching gas are carried out to pattern the fifth
polysilicon layer and to form an upper gate electrode 24 of the TFT.
In FIG.9D, a CVD is carried out to form an insulator layer which is made of
SiO.sub.2 and has a thickness of 1000 .ANG., for example, and an insulator
layer which is made of PSG and has a thickness of 5000 .ANG., for example.
In FIG.9D, these two insulator layers are shown as one insulator layer 25.
Then, a thermal process is carried out to reflow and planarize the
insulator layer 25.
A resist process of the photolithography technique and a RIE using
CHF.sub.3 /He as the etching gas are carried out to selectively etch the
insulator layer 25 and the like and to form a bit line contact hole.
Next, an A1 layer having a thickness of 1 .mu.m, for example, is formed by
a sputtering process, and this A1 layer is patterned using the normal
photolithography technique to form a bit line 26.
According to this embodiment described above with reference to FIGS.9A
through 9D, the concept of the present invention is applied to the double
gate structure TFT load type SRAM, and it can be seen that the number of
mask processes is reduced by two compared to the prior art described with
reference to FIGS.7A through 7C.
In the first and second embodiments described above, the insulator layers
made of SiO.sub.2 and the polysilicon layers are etched during the process
of forming the contact hole 16A shown in FIG.8B and the contact hole 23A
shown in FIG.9B. However, the following problems may occur when
penetrating the stacked structure which is made up of the insulator layers
and polysilicon layers.
FIGS.10A through 10F are side views in cross section showing essential
parts of a semiconductor memory device at essential stages of the
production, for explaining the process of forming a contact hole which
penetrates a stacked structure. In each of FIGS.10A through 10F, the left
hand side shows the result of an expected process while the right hand
side shows the result of an actual process, and the description will be
given mainly with reference to the right hand side of FIGS.10A through 10F
which shows the actual process.
In FIG.10A, a first conductor layer 932 made of polysilicon, a first
insulator layer 933 made of SiO.sub.2, a second conductor layer 934 made
of polysilicon, a second insulator layer 935 made of SiO.sub.2, a third
conductor layer 936 made of polysilicon and a third insulator layer 937
made of SiO.sub.2 are successively formed and stacked on a Si
semiconductor substrate 931.
In FIG.10B, a resist process of the normal photolithography technique and a
RIE using CHF.sub.3 /He as the etching gas are carried out to etch the
third insulator layer 937 and to form a part of a contact hole. In
FIG.10B, the illustration of a photoresist layer is omitted for the sake
of convenience. The illustration of the photoresist layer will be omitted
similarly in FIGS.10C through 10F which follow.
When etching the third insulator layer 937, a part of the underlying third
conductor layer 936 is also etched. As a result, the remaining third
conductor layer 936 becomes extremely thin when the third conductor layer
936 is thin to start with.
In FIG.10C, a RIE using CCl.sub.4 /O.sub.2 as the etching gas is carried
out to etch the third conductor layer 936.
Although dependent on the thickness of the underlying second insulator
layer 935, the thickness of the second insulator layer 935 becomes
extremely thin when the third conductor layer 936 is etched. In an extreme
case, not only the second insulator layer 935 but also a part of the
second conductor layer 934 may be etched when etching the third conductor
layer 936.
In FIG.10D, a RIE using CHF.sub.3 /He as the etching gas is carried out to
etch the second insulator layer 935. However, although this etching is
originally intended to etch the second insulator layer 935, the second
insulator layer 935 may not exist from the start of this etching, for the
reasons described above.
If the second insulator layer 935 is extremely thin or does not even exist
at the start of this etching, the second conductor layer 934 and the first
insulator layer 933 are etched although this etching is originally
intended to etch the second insulator layer 935. When the second conductor
layer 934 and the first insulator layer 933 are etched, the surface of the
first conductor layer 932 may become exposed by this etching which is
originally intended to etch the second insulator layer 935.
In FIG.10E, a RIE using CCl.sub.4 /O.sub.2 as the etching gas is carried
out to etch the second conductor layer 934.
However, although this etching is originally intended to etch the second
conductor layer 934, the surface of the first conductor layer 932 may
already be exposed as described above. In this case, even a part of the Si
semiconductor substrate 931 may be etched by this etching which is
originally intended to etch the second conductor layer 934.
In FIG.10F, a RIE using CHF.sub.3 /He as the etching gas is carried out to
etch the first insulator layer 933.
But although this etching is originally intended to etch the first
insulator layer 933, a part of the Si semiconductor substrate 931 may
already be etched as described above. In this case, the contact hole
completely extends into the Si semiconductor substrate 931 as shown in
FIG.10F and the Si semiconductor substrate 931 is extremely damaged.
If the contact hole extends into the Si semiconductor substrate 931 as
shown in FIG.10F, a leak is introduced at a node part between the driver
transistor and the load transistor of the memory cell, and the operation
of the memory cell becomes unstable. Hence, it is conceivable to carry out
a moderate etching so as to etch only the originally intended layer, but
such a moderate etching is difficult to control.
The etching of each layer cannot be controlled to etch only the originally
intended layer by the moderate etching, because the contact hole is seldom
formed at a flat part shown in FIGS.10A through 10F. In actual practice,
the contact hole is usually formed at a stepped part, and an etching
residue tends to remain at the stepped part within the contact hole.
However, a sufficient over-etching becomes necessary in order to remove
such an etching residue.
Next, a description will be given of a third embodiment of the
semiconductor memory device according to the present invention which is
produced by a third embodiment of the method of producing the
semiconductor memory device according to the present invention. FIGS.11A
through 11F are side views in cross section showing an essential part of
the third embodiment of the semiconductor memory device according to the
present invention at essential stages of the third embodiment of the
method of producing the semiconductor memory device according to the
present invention, for explaining the operating principle of the third
embodiment. In each of FIGS.11A through 11F, the left hand side shows the
result of an expected process while the right hand side shows the result
of an actual process, and the description will be given mainly with
reference to the right hand side of FIGS.11A through 11F which shows the
actual process.
In this embodiment, measures are taken so that the contact hole will not
extend into the substrate even when an over-etching is carried out to
remove the etching residue within the contact hole.
In FIG.11A, a first conductor layer 142 made of polysilicon, a second
conductor layer 143 made of a refractory metal such as W and Mo or a
silicide thereof, a first insulator layer 144 made of SiO.sub.2, a third
conductor layer 145 made of polysilicon, a second insulator layer 146 made
of SiO.sub.2, a fourth conductor layer 147 made of polysilicon and a third
insulator layer 148 made of SiO.sub.2 are successively formed and stacked
on a Si semiconductor substrate 141.
In FIG.11B, a resist process of the normal photolithography technique and a
RIE using CHF.sub.3 /He as the etching gas are carried out to etch the
third insulator layer 148 and to form a part of a contact hole. In
FIG.11B, the illustration of a photoresist layer is omitted for the sake
of convenience. The illustration of the photoresist layer will be omitted
similarly in FIGS.11C through 11F which follow.
During this process, a part of the fourth conductor layer 147 underlying
the third insulator layer 148 is also etched. Accordingly, the thickness
of the fourth conductor layer 147 remaining after this etching is also
extremely small, similarly as in the case described above with reference
to FIGS.10A through 10F.
In FIG.11C, a RIE using HBr/Ar as the etching gas is carried out to etch
the fourth conductor layer 147.
In this case, although dependent on the thickness of the second insulator
layer 146 underlying the fourth conductor layer 147, the thickness of the
second insulator layer 146 would be reduced by this etching. In an extreme
case, not only the second insulator layer 146 but also a part of the third
conductor layer 145 becomes etched by this etching which is originally
intended to etch the fourth conductor layer 147.
In FIG.11D, a RIE using CHF.sub.3 /He as the etching gas is carried out to
etch the second insulator layer 146.
However, the thickness of the second insulator layer 146 is already reduced
to start with and may not even exist in an extreme case, as described
above. Hence, although this etching is originally intended to etch the
second insulator layer 146, the third conductor layer 145 and the first
insulator layer 144 may be etched to expose the surface of the second
conductor layer 143.
In FIG.11E, a RIE using HBr/Ar as the etching gas is carried out to etch
the third conductor layer 145.
Even though the surface of the second conductor layer 143 may already be
exposed as described above when this etching is carried out, the second
conductor layer 143 is made of the refractory metal or refractory metal
silicide. For this reason, the second conductor layer 143 is hardly etched
by this RIE which uses HBr, and no problems will be introduced by this
RIE.
In FIG.11F, a RIE using CHF.sub.3 /He as the etching gas is carried out to
etch the first insulator layer 144.
The first insulator layer 144 may already be etched when this etching is
started, but no adverse effects are introduced because the second
conductor layer 143 is made of the refractory metal or refractory metal
silicide and is hardly etched by the CHF.sub.3 /He etching gas.
Therefore, the contact hole is formed to an ideal shape. The only
difference of the resulting contact hole shown in FIG.11F with that shown
in FIG.10F is that the lowermost conductor layer in FIG.11F is made up of
the first conductor layer 142 and the second conductor layer 143.
Next, a more detailed description will be given of the third embodiment of
the semiconductor memory device according to the present invention and the
third embodiment of the method of producing the semiconductor memory
device according to the present invention, by referring to FIGS.12A
through 12H. FIGS.12A through 12H are side views in cross section showing
an essential part of the third embodiment of the semiconductor memory
device according to the present invention at essential stages of the third
embodiment of the method of producing the semiconductor memory device
according to the present invention. In this embodiment, the present
invention is applied to the TFT load type SRAM.
In FIGS.12A through 12H, those parts which are the same as those
corresponding parts in FIGS.1A through 1F and FIGS.4A through 4D are
designated by the same reference numerals, and a description thereof will
be omitted. In this embodiment of the method, the processes are the same
as those of the prior art method up to the processes shown in FIGS.1A
through 1F, that is, until the gate insulator layer 3 is selectively
etched to form the contact hole 3A. Hence, a description will only be
given of the processes carried out thereafter. In the following
description, the prior art processes descried with reference to FIGS.4A
through 4D will help the understanding of this embodiment.
In FIG.12A, it is assumed that the field insulator layer 2, the gate
insulator layer 3 and the contact hole 3A are already formed on the Si
semiconductor substrate 1.
A CVD is carried out to form a first polysilicon layer having a thickness
of 1000 .ANG., for example. This first polysilicon layer corresponds to
the first conductor layer.
By carrying out a vapor phase diffusion, P is introduced with an impurity
concentration of 1.times.10.sup.21 cm.sup.-3 to form an n.sup.+ -type
impurity region 5'.
A CVD is carried out to form a WSi layer 150 having a thickness of 1000
.ANG., for example. This WSi layer 150 corresponds to the second conductor
layer. The material used for the layer 150 is not limited to WSi, but
other refractory metals such as W or refractory metal silicides may be
used in place of WSi.
In FIG.12B, a resist process of the photolithography technique and a RIE
using CCl.sub.4 /O.sub.2 as the etching gas for etching WSi and
polysilicon are carried out to pattern the WSi layer 150 and the first
polysilicon layer and to form a gate electrode 4. This gate electrode 4 is
the gate electrode of the driver transistor and the word line.
An ion implantation is carried out to inject As ions with a dosage of
3.times.10.sup.15 cm.sup.-2 and an acceleration energy of 40 keV, so as to
form a source region 5 and a drain region 6.
In FIG.12C, a CVD is carried out to form an insulator layer 7 which is made
of SiO.sub.2 and has a thickness of 1000 .ANG., for example.
A resist process of the photolithography technique and a RIE using
CHF.sub.3 /He as the etching gas are carried out to form a ground line
contact hole. This ground line contact hole cannot be shown in FIG.12C,
but this ground line contact hole may be understood from FIG.2B, for
example.
In FIG.12D, a CVD is carried out to form a second polysilicon layer which
has a thickness of 1500 .ANG., for example.
In addition, an ion implantation is carried out to inject P ions into the
second polysilicon layer.
A resist process of the photolithography technique and a RIE using
CCl.sub.4 /O.sub.2 as the etching gas are carried out to pattern the
second polysilicon layer and to form a ground line 8.
In FIG.12E, a CVD is carried out to form an insulator layer 9 which is made
of SiO.sub.2 and has a thickness of 1000 .ANG., for example, on the entire
top surface of the stacked structure shown in FIG.12D.
A CVD is carried out to form a third polysilicon layer which has a
thickness of 500 .ANG., for example.
An ion implantation is carried out to inject P ions with a dosage of
1.times.10.sup.15 cm.sup.-2 and an acceleration energy of 10 keV.
A resist process of the photolithography technique and a RIE using
CCl.sub.4 /O.sub.2 as the etching gas are carried out to pattern the third
polysilicon layer and to form a gate electrode 15 of the TFT.
In FIG.12F, a CVD is carried out to form a gate insulator layer 16 which is
made of SiO.sub.2 and has a thickness of 200 .ANG., for example.
A resist process of the photolithography technique and RIEs using CHF.sub.3
/He and HBr/Ar as the etching gases for respectively etching SiO.sub.2 and
polysilicon are carried out, to selectively etch the gate insulator layer
16, the gate electrode 15 which is made of the third polysilicon layer,
the insulator layer 9 and the insulator layer 7, and to form a contact
hole 16A. This contact hole 16A extends from the surface of the gate
insulator layer 16 and reaches the gate electrode 4 of the driver
transistor. This process forms an essential part of this embodiment. Even
if a sufficient over-etching is carried out so that no etching residue
remains within the contact hole 16A after the etching, no adverse damage
is made to the surface of the underlying WSi layer 150 which forms the
surface of the gate electrode 4. Hence, no damage is made to the first
polysilicon layer and the surface of the Si semiconductor substrate 1 by
this over-etching.
In FIG.12G, a CVD is carried out to form a fourth polysilicon layer having
a thickness of 200 .ANG., for example.
A resist process of the photolithography technique and an ion implantation
are carried out to inject B ions with a dosage of 1.times.10.sup.14
cm.sup.-2 and an acceleration energy of 5 keV into parts where source and
drain regions of the TFT are formed.
A resist process of the photolithography technique and a RIE using
CCl.sub.4 /O.sub.2 as the etching gas are carried out to pattern the
fourth polysilicon layer and to form a source region 17, a drain region 18
and a channel region 19 of the TFT, a Vcc supply line and the like. The
Vcc supply line cannot be shown in FIG.12G, but the Vcc supply line may be
understood from FIG.2F or 5D.
In FIG.12H, a CVD is carried out to form an insulator layer which is made
of SiO.sub.2 and has a thickness of 1000 .ANG., for example, and an
insulator layer which is made of PSG and has a thickness of 5000 .ANG.. In
FIG.12H, these two insulator layers are shown as one insulator layer 21 in
FIG.12H, similarly as in the case of FIG.4D.
Then, a thermal process is carried out to reflow and planarize the
insulator layer 21.
A resist process of the photolithography technique and a RIE using
CHF.sub.3 /He as the etching gas are carried out to selectively etch the
insulator layer 21 and to form a bit line contact hole.
A sputtering is carried out thereafter to form an A1 layer having a
thickness of 1 .mu.m, for example, and this A1 layer is patterned using
the normal photolithography technique so as to form a bit line 22.
According to this embodiment described with reference to FIGS.12A through
12H in particular, only one mask process is required to achieve contact of
the gate electrode of the driver transistor, the gate electrode of the TFT
load and the drain of the TFT load. In addition, when forming the contact
hole for achieving this contact, there is no possibility of the underlayer
and the substrate from becoming damaged and the characteristic of the
memory cell from deteriorating even when the over-etching is carried out
to generate no etching residue within the contact hole. On the other hand,
in the prior art shown in FIGS.4 and 5, two mask processes are required to
achieve the above described contact.
Next, a description will be given of a fourth embodiment of the
semiconductor memory device according to the present invention and a
fourth embodiment of the method of producing the semiconductor memory
device according to the present invention, by referring to FlGS.13A
through 13D. FIGS.13A through 13D are side views in cross section showing
an essential part of the fourth embodiment of the semiconductor memory
device according to the present invention at essential stages of the
fourth embodiment of the method of producing the semiconductor memory
device according to the present invention. In this embodiment, the present
invention is applied to the double gate structure TFT load type SRAM.
In FIGS.13A through 13D, those parts which are the same as those
corresponding parts in FIGS.12A through 12H are designated by the same
reference numerals, and a description thereof will be omitted. In this
embodiment of the method, the processes are the same as those of the third
embodiment of the method up to the processes shown in FIGS.12A through
12E, that is, until the gate electrode 15 of the TFT is formed from the
third polysilicon layer. Hence, a description will only be given of the
processes carried out thereafter. In the following description, the prior
art processes descried with reference to FIGS.7A through 7C will help the
understanding of this embodiment.
In FIG.13A, a double gate structure TFT load type SRAM includes a Si
semiconductor substrate 1, a field insulator layer 2 formed on the Si
semiconductor substrate 1, a gate insulator layer 3, a gate electrode 4 of
the driver transistor which is made up of the first polysilicon layer and
a WSi layer 150 (second conductor layer), an n.sup.+ -type source region
5, an n.sup.+ -type drain region 6, an n.sup.+ -type diffusion region 6',
an insulator layer 7, a ground line 8 which is made up of the second
polysilicon layer, a lower gate electrode 15 of the TFT, and a gate
insulator layer 16 of the TFT.
A CVD is carried out to form a fourth polysilicon layer which has a
thickness of 200 .ANG., for example.
A resist process of the photolithography technique and an ion implantation
are carried out to inject B ions with a dosage of 1.times.10.sup.14
cm.sup.-2 and an acceleration energy of 5 keV into parts where source and
drain regions of the TFT are formed.
A resist process of the photolithography technique and a RIE using
CCl.sub.4 /O.sub.2 as the etching gas are carried out to pattern the
fourth polysilicon layer and to form a source region 17, a drain region 18
and a channel region 19 of the TFT and a Vcc supply line. The Vcc supply
cannot be seen in FIG.13A.
In FIG.13B, a CVD is carried out to form an insulator layer 23 which is
made of SiO.sub.2 and has a thickness of 500 .ANG., for example.
A resist process of the photolithography technique and RIEs using CHF.sub.3
/He and HBr/Ar as the etching gases for respectively etching SiO.sub.2 and
polysilicon are carried out to selectively etch the insulator layer 23,
the drain region 18 of the TFT formed from the fourth polysilicon layer,
the gate insulator layer 16, the gate electrode 15 which is formed from
the third polysilicon layer, the insulator layer 9 and the insulator layer
7, and to form a contact hole 23A. This contact hole 23A extends from the
surface of the insulator layer 23 and reaches the gate electrode 4 of the
driver transistor. The gate electrode 4 is made up of the WSi layer 150
and the first polysilicon layer. This process forms an essential part of
this embodiment.
In FIG.13C, a CVD is carried out to form a fifth polysilicon layer which
has a thickness of 500 .ANG., for example.
Then, a thermal diffusion is carried out to diffuse P into the fifth
polysilicon layer with an impurity concentration of 1.times.10.sup.21
cm.sup.-3, for example.
A resist process of the photolithography technique and a RIE using
CCl.sub.4 /O.sub.2 as the etching gas are carried out to pattern the fifth
polysilicon layer and to form an upper gate electrode 24 of the TFT.
In FIG.13D, a CVD is carried out to form an insulator layer which is made
of SiO=hd 2 and has a thickness of 1000 .ANG., for example, and an
insulator layer which is made of PSG and has a thickness of 5000 .ANG.,
for example. In FIG.13D, these two insulator layers are shown as one
insulator layer 25.
Thereafter, a thermal process is carried out to reflow and planarize the
insulator layer 25.
A resist process of the photolithography technique and a RIE using
CHF.sub.3 /He as the etching gas are carried out to selectively etch the
insulator layer 25 and the like and to form a bit line contact hole.
Next, a sputtering is carried out to form an A1 layer having a thickness of
1 .mu.m, for example, and this A1 layer is patterned using the normal
photolithography technique so as to form a bit line 26.
In this embodiment, the concept shown in FIGS.11A through 11F is applied to
the double gate structure TFT load type SRAM. When forming the contact
hole for achieving the necessary contact, there is no possibility of the
underlayer and the substrate from becoming damaged and the characteristic
of the memory cell from deteriorating even when the over-etching is
carried out to generate no etching residue within the contact hole.
Compared to the prior art method described with reference to FIGS.7A
through 7C, the number of mask processes which are required to achieve the
above described contact in this embodiment is reduced by two.
Further, the present invention is not limited to these embodiments, but
various variations and modifications may be made without departing from
the scope of the present invention.
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